Conventionally, a rotary pump such as that disclosed in Japanese Patent Publication Laid-Open No. 2000-179466 has been used as an internal gear type rotary pump, such as a trochoid pump. The rotary pump includes an inner rotor having an external tooth portion in its outer periphery, an outer rotor having an internal tooth portion in its inner periphery, and a casing that houses the inner rotor and the outer rotor. The inner rotor and the outer rotor are disposed within the casing such that the internal teeth portion and the external teeth portion mesh with each other and a plurality of chambers are formed between the respective tooth portions.
If a line which passes through central axes of both the inner rotor and the outer rotor is defined as a center line of the pump, an intake port and a discharge port that communicate with the plurality of chambers are provided on either side of the center line. When the pump is driven, the central axis of the inner rotor is set as the drive shaft, and the inner rotor is rotated via the drive shaft. Accompanying the rotation of the inner rotor, the outer rotor is also rotated in the same because the inner tooth portion and the outer tooth portion mesh each other. At this time, the pump intakes brake fluid from the intake port, and discharges brake fluid from the discharge port due to respective volumes of the plurality of chambers becoming larger and smaller during each rotation of the outer rotor and the inner rotor.
Sealing of both end surfaces in an axial direction of the rotary pump is achieved by a resin member made of resin. The resin member functions as a seal by being pushed by an elastic member made from an elastic body such as rubber, or the like.
However, when a sealing manner utilizing such a resin made sealing unit is adopted for both end surfaces in the axial direction, as in the above rotary pump, leads to an increase in cost. Accordingly, it is possible to reduce cost with a structure that adopts the resin made sealing unit at one end surface of the rotary pump, and a mechanical seal in which the inner rotor and the outer rotor are directly pressed against a side plate at the other end surface of the rotary pump.
With above mentioned structure, however, if the mechanical seal is adopted, the inner rotor and the outer rotor, which are both made of metal, are forcefully pressed against the metal made side plate, and thus a problem is caused in which drive torque increases substantially due to friction resistance becoming larger.
Further, since a contact area of the inner rotor or the outer rotor and the side plate is large, a problem occurs in which drive torque increases substantially as a result of shear resistance of brake fluid, particularly when viscosity of the brake fluid becomes high at low temperatures.